The present invention generally relates to data reading apparatuses for data transmission, and more particularly to a data reading apparatus capable of accurately reading digital data even if jitter exists in the digital data.
When transmitting an analog information signal, the analog information signal is subjected to digital pulse modulation and converted into a digital data. The digital data is divided into predetermined sections, and a signal in each predetermined section is added with a synchronizing signal having a fixed pattern, and often further added with a code error detection signal and a code error correction signal. Such a digital data divided into the predetermined sections and added with the above signals, is transmitted time serially. The digital signal series is subjected to modulation such as frequency modulation and phase modulation, and transmitted to a transmission path such as a magnetic tape. The digital signal series thus obtained through the transmission path, is supplied to a receiving apparatus (or a reproducing apparatus). If the received signal is modulated upon transmission, the receiving apparatus first demodulates the received signal, and waveform equalization is performed at an equalizer so that the total impulse response becomes zero in the Nyquist interval. The signal thus subjected to waveform equalization at the equalizer so that intersymbol interference does not occur, is supplied to a level comparator (detector) wherein the signal level is compared with a reference level and converted into a two-valued digital signal series. The two-valued digital signal series is supplied to a synchronizing signal detecting circuit and a data reading apparatus.
The above synchronizing signal detecting circuit detects a synchronizing signal within the digital signal series, and supplies an output detection signal to the data reading apparatus. The data reading apparatus comprises a reference clock generator, a data reading timing clock generator, and a data reading circuit. This data reading apparatus reads the digital data transmitted in succession to the above synchronizing signal at the data reading circuit, by a data reading timing clock pulse formed from the above synchronizing signal detection signal and a reference clock pulse obtained from the reference clock generator.
The reference clock generator generates a reference clock pulse having a transmission bit rate N, where N is an integer greater than or equal to two. Because the reference clock pulse and the transmitted digital data are in a non-synchronous relationship, the data reading timing clock pulse formed by counting the reference clock pulse and the digital data gradually become out of phase if jitter and the like introduced in the transmission path exists within the digital data. Hence, in the conventional data reading apparatus, the data reading clock pulse becomes generated at a point when there is an incoming digital data of a bit adjacent to the bit which is to be read originally, when the data reading timing clock pulse and the digital data become out of phase to a certain extent. In such a case, the data reading apparatus reads the digital data of the bit adjacent to the bit which is to be read originally, and there was a disadvantage in that erroneous data reading operations could occur.